For the Record: On American Armour

After my articles on Soviet and German armour, I think it’s only fair to sail across the Atlantic*and turn our eyes at the manufacturing juggernaut that was the United States. A lot of people know that the US was good at casting armour, and that their armour was softer than that of their European counterparts, but that’s about it. While this is true, it’s far from the whole story. To begin, let’s go far far back to the 1920s. There are no Shermans, Tigers, or T-34s. The greatest war mankind has ever known has just ended, and everyone is busy developing the latest and greatest killing machine. Huge and slow monsters did not seem interesting to the US Army, and development was focused on vehicles that were quick and agile, but paid for that agility with thickness of armour. For instance, here is an attempt to produce 6-12 mm armour plates that protect against .30 and .50 caliber bullets in 1922. Maximum hardness!? This isn’t the American armour we know and love! Don’t worry, they have two decades to get there. In the meantime, let’s see how well thin armour like this turns out. Well that’s not good. To be fair, the unrolled plates didn’t crack, but were covered in free cementite, which was what the metallurgists were trying*to avoid. Ok, so the early armour wasn’t of amazing quality. How well did it resist attack? A document about corrugated armour conveniently mentions the ballistic limits for flat armour defined by specification AXS54 Rev. 2 in a table. We see that the 12.7 mm plate can be penetrated by a .30 caliber bullet at a speed of 2250 f/s*and a 14 mm plate can be penetrated by a .50 cal bullet at 1930 f/s. Both of those bullets were very capable of achieving*those speeds, so early American vehicles were still vulnerable to small arms fire. I suppose it’s worth mentioning that Soviet vehicles with similar armour thicknesses were not vulnerable to .30 caliber bullets, but that was a few years later. All right, so the armour can be penetrated by bullets. Does it still spall? The Americans invented a pretty sweet process for taking pictures of rapidly moving things, which indicated that yes, yes it did. Oh dear. Let’s fast forward to 1938, a year before the world erupts into total war once more. American armour has improved to this point. It got a little softer (400 BNH ideal hardness) and a little thicker, now expecting protection from proper 37 mm guns. 1700 m/s is the ballistic limit of an inch of armour against a 37 mm gun. This gives protection from 37 mm AP from about 850 meters. To put this value into perspective, English reports indicate that the American 37 mm guns penetrate 30 mm of German armour from 1600 yards (1463 meters). This is very high quality armour. Also note that the armour no longer spalls when hit with .50 cal bullets. These values are satisfactory against the current German army, but they won’t be limited to PzIs, PzIIs, and PzIIIs with 37 mm guns forever. The Americans needed more tanks and needed them quickly. How do you get more tanks quickly? Casting! 119 cast armour plates from 9.5 mm to 76 mm were tested in 1940 in order to figure out how well this sort of thing can be done. The report only mentions that “some” plates passed the specifications, but fails to mention how many.*Another report, however, does mention some pretty serious issues with casting. Yikes, that’s not a good thing to hear when you’re about to participate in a huge war. This is where the Americans rapidly started leaning towards softer armour. Watch as the hardness falls from 400 BNH to only 255! Also, note the superiority of Canadian manufactured armour. Take that, Americans! Let’s look at slightly thicker armour. A document on welding provides a table of ballistic limits of 1.5-2 inch plates with and without weld repairs. Not only is the ballistic limit of a plate with a weld not that much less than an undamaged plate (remember all those popping German seams?), it’s only decreased when the shell strikes within an inch of the seam. This is pretty damn good. Furthermore, the ballistic limit of a 2 inch plate is 1512-1640 f/s, which corresponds to 500-900 meters (at least according to Soviet tables for the M3 gun). Recall that both Soviet and German*data indicates that the Tiger can be penetrated in the side from the outer limit of that range. The quality of American armour far surpasses the quality of German armour, even though the plate is overmatched in the American case. While this assertion is only backed by theory, battlefield performance seems to agree. Damage to vehicles consists mainly of broken tracks, tank fires, broken turret rings and damaged suspension system. Apparently armour plate quality superior to that of Germans. While the cast turret rings seem to still be a bit of a problem, the armour shows itself splendidly. No observed complete penetration to front sloping plate, front tank doors, nor gun shields. Final drive housing struck by what is believed to be 6-pounder armor piercing projectile was dented with no effect on operation.* One penetration reported by armor piercing 6-pounder on edge of door next to T member, numerous penetrations of side plates and back plates with no effect on operation of vehicles. And this isn’t even the Sherman we’re talking about, but the meek M3 Stuart.* My communist benefactors are looking over my shoulder, so I have to say something negative about American armour now. Uh, let’s see here… Ha! The tanks are falling apart in winter! How cold did that*winter get, anyway? Oh, pretty damn cold, huh. Sadists can read some of the detailed descriptions of the damage in extremely cold temperatures (1, 2, 3, 4, 5, 6), but for those of you that don’t have time, the damage is very similar to that of German vehicles at room temperatures: cracking, spalling, fragments falling off. So there you have it.*Both cast and soft armour came from necessity instead of some kind of inherent superiority, but the Americans pulled it off. The quality of armour sent to Europe and the Pacific was largely excellent, and definitely superior to the armour of the vehicles it was fighting against.